Hi all and greetings from the Bahamas.
My question is: Has any work been done recently on paddlewheel propulsion?
I am aware that the tug-of-war test done by the British Admiralty in the 1800's (HMS Rattler etc.) proved the screw to be superior to paddles but it seems to me that modern paddlewheels may be able to compete again.
Props are referred to as "lift" devices as opposed to traditional paddles (or oars, for that matter) as "drag" devices. But those are low speed devices. If paddle wheels were made from high-speed Pelton wheels, for example, they would become "reaction" devices and the rules of the game would change dramatically.
A Pelton wheel has cupped blades that almost completely reverse the direction of an impinging water jet, thereby captuing most of the momentum of that jet. Efficiency is high, if I recall correctly, because the wetted surface is low. The maximum power delivered when the cups move at half the velocity of the jet.

Converting all this to boat design, why not have a boat with a rear "axle" like a pickup having a Pelton wheel protruding out on each side with the lower part in the water. The engine could have an automotive 4 speed and reverse gear box. The tip speed of the wheels would be twice the cruising speed. On startup, as the wheels dug in, the stern of the boat should rise, unlike a propellor system. The water would be hurled out of the back (need some kind of fenders?) like a jet pump. So here is a surface piercing reaction paddle wheel. Would it work?

Now, if you really want to have fun, make a 4-wheel drive version and fly the entire boat like a hydrofoil. With long-travel suspension on the wheels, the thing could be awesome in chop! Whadjathink?
Will Jones

Looks to me like the pelton wheel might retain a lot of water and greatly reduce the efficency in this mode. Interesting thought though. Ordinarily, the energy in the water jet would (along with gravity) causes it to exit the scoops but in the powered mode, the extra energy (1 - %eff.) would cause water to stay in the scoop.

I like Tom's analysis. Although an anemometer is great for measuring wind speed, it is useless for powering aircraft (OK, OK - bad analogy, but it is the same kind of shape...)

If there is any water left in the Pelton cups, which there may well be even after centrifugal "force" has done its thing, then the whole reason for having cups goes out the window.

The British tug-of-war may have used an old paddle-wheel, but it also used an old propeller... ;-) Very old by today's technology. If you wanted to put the same amount of time into developing efficient paddle-wheels, I'm sure it could be done, but there simply hasn't been the interest. Got a spare 100 years?

Hmmm! maybe you are right, Tom. The shape of the cups would be critical.
I looked up the US Patent Office records and came up with this design. (Attachment only; sorry, I can't post drawings yet ). The treads on the paddle wheels are straight so the water may eject more cleanly.
Will Jones

After another look at the pelton bucket wheel idea, I remembered that the (nearly) same design is often used in many parts of the world to lift water to a higher level. This makes me think that my original objection may be valid.

Gonzo: Yes, that would probaly help, but add some complexity. Old time paddle wheels were sometimes "feathered" to keep the paddles vertical during their power stroke. It would also help to have "sides" on the paddles to prevent lateral water flow; in other words, cupped in only one plane. The major part of the design problem is the shape of the paddles, I think.

Tom: If you mean the Far East water wheels that are powered by large buckets dragging in the water stream, I don't think we can learn much from them. The point of a "reverse pelton wheel" is that it is a high speed "reaction" device and those water wheels are basically low speed drag devices.

Since the basic requirement is to pick up water at zero velocity and throw it out horizontally at the stern at a higher velocity, it would be possible to use a partial fixed housing (like a European undershot water wheel or the volute of a centrifugal pump). But that solution would not provide lift for the boat. Even if the reverse pelton wheel were not as efficient as, say, an Arneson surface piercing drive, it might gain overall by reducing hull drag.

Question: How efficient is a propellor and how is efficiency measured? (I bet props - especially small diameter ones - spill a lot of water over the tips of the blades by centrifuga force).
Merry Christmas, all!
Will Jones

PS: Why do the big Aussie catamaran ferries with jet drives fire the water out ABOVE the water line? That surprised me when I witnessed it recently; I bet there is a good reason.

Guest says:
"PS: Why do the big Aussie catamaran ferries with jet drives fire the water out ABOVE the water line? That surprised me when I witnessed it recently; I bet there is a good reason."

Because there is too much drag if you shoot it out underwater. Seriously, dammit! A waterjet is a real jet, and relies on the acceleration at the nozzle to provide thrust, not just moving mass from one place to another. If you shoot the jet out into "solid" watyer, it is immediately slowed down by having to move the surrounding water, which reduces thrust.
Steve

Without a drawing of your original proposal, it is bit hard to get a complete understanding across but I assumed that you were operating the pelton wheel in the same way that a normal paddle wheel would operate.

I did not mean the far eastern wheel you mentioned. I am referring to the ones that use human power to lift water to a higher level, usually for irrigation purposes. I don't know how closely this analogy follows though.

One measure of propeller efficiency is the fraction of slip. That is, the actual forward advance by the propeller divided by the propeller pitch. If a prop has a pitch of 12" and advances 9" for each rotation, then the slip is 25% and the efficiency would be 75%. Efficiency generally increases with speed.

Some propellers operate with slip percentage in the single digits. I can't imagine any kind of paddle wheel doing that well but I'm no expert.

Steve,
Re your explanation on water jets, I can't find any support for it. Obviously, there must be some reason the big cats discharge the water jets above the water line, but no one else seems to do it that way. For example this link implies the lower the jet the better.

It may improve efficiency slightly but maybe it is just to keep the bow down. On the other hand, maybe the jets I saw coming out of the stern were not the main thrusters - but they sure looked big!
Perhaps our Tasmanian members can comment.
Will Jones

Will,
Thanks (I think!)
If you look at most jet boats, the jets ARE below water at rest and at low speeds, but once the boat starts planing, the jets will be out of the water. No other way to do it on a planing boat ;-)
Stegve